Laser-induced thermal transfer processes are well-known in applications such as color proofing and lithography. Such laser-induced processes include, for example, dye sublimation, dye transfer, melt transfer, and ablative material transfer. These processes have been described in for example, Baldock, UK Patent 2,083,726; DeBoer, U.S. Pat. No. 4,942,141; Kellogg, U.S. Pat. No. 5,019,549; Evans, U.S. Pat. No. 4,948,776; Foley et al., U.S. Pat. No. 5,156,938; Ellis et al., U.S. Pat. No. 5,171,650; and Koshizuka et al., U.S. Pat. No. 4,643,917.
Laser-induced processes use a laserable assemblage comprising (a) a donor element that contains the imageable component, i.e., the material to be transferred, and (b) a receiver element. The donor element is imagewise exposed by a laser, usually an infrared laser, resulting in transfer of material to the receiver element. The exposure takes place only in a small, selected region of the donor at one time, so that the transfer can be built up one pixel at a time. Computer control produces transfer with high resolution and at high speed.
For the preparation of images for proofing applications, the imageable component is a colorant. For the preparation of lithographic printing plates, the imageable component is an oleophilic material which will receive and transfer ink in printing.
Hotta et al., U.S. Pat. No. 4,541,830, disclose the inclusion of nonsublimable particles in the dye layer of a dye transfer sheet used in a dye sublimation process. In a dye sublimation transfer process, the material being transferred is a gas, i.e., the subliming dye. DeBoer, U.S. Pat. No. 4,772,582, discloses that a separate layer of "spacer beads" should be used in such transfer elements.
A dye sublimation process is quite different from a laser ablative transfer process. In a dye sublimation process, an imageable component is converted into gaseous form and transferred via condensation onto the receiver surface. In an ablative transfer process, an imageable component is transferred as a solid material by an explosive force onto the receiver element. The mechanisms by which the transfer is effected are very different in the two processes. Factors which improve transfer in one process will not necessarily be applicable in the other process. As previously mentioned, such processes have been described in, e.g., Foley et al., U.S. Pat. No. 5,156,938, and Ellis et al., U.S. Pat. No. 5,171,650. These processes are fast and result in transfer of material with high resolution. However, it has been found that the solid image uniformity is frequently poor. Large solid images have a mottled or striated appearance which is generally unacceptable in proofing applications and in the printing industry.